Atherosclerotic plaque deposits tend to occlude and thus restrict the f low of blood in coronary arteries. Such deposits are a major cause of heart disease and various techniques have been proposed to remove atherosclerotic plaque without requiring surgery.
The use of lasers for the purpose of evaporating (ablating) atherosclerotic plaque deposits has been proposed and is currently under investigation by a number of researchers. The procedure (known as angioplasty) involves delivery of laser energy through an optical fiber to the site at which ablation must occur. While laser angioplasty is potentially advantageous, it has proven difficult to control accurately and reliably the process so as to ensure that only atherosclerotic plaque is ablated. Accidental perforation of an artery during a laser ablation process can be fatal.
A variety of techniques have been proposed to position accurately the tip of the optical fiber through which the laser energy is transmitted with respect to the plaque that it is desired to ablate. Among other things, accurate positioning of the optical fiber requires diagnostic procedures for determining the presence of plaque deposits and the proper positioning of the end of the fiber with respect to such deposits so that only that portion of the tissue diagnosed as plaque will be ablated by the high power laser energy. See, for example, Deckelbaum U.S. Pat. No. 4,785,806, and Mok U.S. Pat. No. 4,641,650. If only plaque deposits are subjected to the high energy laser, the likelihood of accidental perforation of the artery is reduced.
Ultrasonic diagnosis has been proposed also to avoid possible perforation of an arterial wall during laser angioplasty. In such systems, ultrasonic pulses are transmitted toward the arterial wall and the time of arrival of the echoes from the tissue interfaces measured. Knowing the velocity of sound, the distance (range) of each tissue layer from the catheter can be calculated to provide a visual image of the cross-section of the arterial wall scanned by the ultrasound. The image reflects the thickness of the arterial wall so that the operator knows not to ablate tissue where the arterial wall is dangerously thin. Secondarily, particularly in the hands of a skilled operator, the cross-sectional image can be used to help distinguish plaque deposits from arterial tissues to ensure that the high energy laser ablation pulses are directed only at the atherosclerotic tissue.
The main object of the present invention is to provide an improved catheter construction for use in laser angioplasty wherein the elements required for delivery of the energy used for range sensing and ablation are contained within the catheter.
A more specific object is to provide an improved catheter construction which includes an ultrasonic transducer for use in laser angioplasty wherein the likelihood of accidental perforation of the arterial wall is reduced.
A further object of the invention is to provide a catheter construction including an ultrasonic transducer for use in laser angioplasty wherein ultrasonic imaging can be achieved with a minimum number of wires, possibly none, thereby reducing space requirements and simplifying the construction.
A still further object of the invention is to provide an ultrasonic imaging system for use in laser angioplasty which has a low space requirement and is easier to install in the catheter.